Sphingolipid metabolism enzymes as targets for anticancer therapy.

Treatment with anti-cancer agents in most cases ultimately results in apoptotic cell death of the target tumor cells. Unfortunately, tumor cells can develop multidrug resistance, e.g., by a reduced propensity to engage in apoptosis by which they become insensitive to multiple chemotherapeutics. Ceramide. the central molecule in cellular sphingolipid metabolism, has been recognized as an important mediator of apoptosis. Moreover, an increased cellular capacity for ceramide glycosylation has been identified as a novel multidrug resistance mechanism. Indeed, virtually all multidrug resistant cell types exhibit a deviating sphingolipid composition, most typically an increased level of glucosylceramide. Thus, the enzyme glucosylceramide synthase, which converts ceramide into glucosylceramide, has emerged as a potential target to increase apoptosis and decrease drug resistance of tumor cells. In addition, several other steps in the pathways of sphingolipid metabolism arc altered in multidrug resistant cells, opening a perspective on additional sphingolipid metabolism enzymes as targets for anti-cancer therapy. In this article, we present an overview of the current understanding concerning drug resistance-related changes in sphingolipid metabolism and how interference with this metabolism can be exploited to over come multidrug resistance.

The involvement of sphingolipids in multidrug resistance.

Administration of most chemotherapeutic agents eventually results in the onset of apoptosis, despite the agents' variety in structure and molecular targets. Ceramide, the central molecule in cellular glycosphingolipid metabolism, has recently been identified as an important mediator of this process. Indeed, one of the events elicited by application of many cytotoxic drugs is an accumulation of this lipid. Treatment failure in cancer chemotherapy is largely attributable to multidrug resistance, in which tumor cells are typically cross-resistant to multiple chemotherapeutic agents. Different cellular mechanisms underlying this phenomenon have been described. Of these the drug efflux pump activity of P-glycoprotein and the multidrug resistance-associated proteins are the most extensively studied examples. Recently, an increased cellular capacity for ceramide glycosylation has been recognized as a novel multidrug resistance mechanism. Indeed, virtually all multidrug-resistant cells exhibit a deviating sphingolipid composition, most typically, increased levels of glucosylceramide. On the other hand, several direct molecular interactions between sphingolipids and drug efflux proteins have been described. Therefore, in addition to a role in the multidrug resistance phenotype by which ceramide accumulation and, thus, the onset of apoptosis are prevented, an indirect role for sphingolipids might be envisaged, by which the activity of these efflux proteins is modulated. In this review, we present an overview of the current understanding of the interesting relations that exist between sphingolipid metabolism and multidrug resistance.

Induction of apoptosis through B-cell receptor cross-linking occurs via de novo generated C16-ceramide and involves mitochondria.

B-cells, triggered via their surface B-cell receptor (BcR), start an apoptotic program known as activation-induced cell death (AICD), and it is widely believed that this phenomenon plays a role in the restriction and focusing of the immune response. Although both ceramide and caspases have been proposed to be involved in AICD, the contribution of either and the exact molecular events through which AICD commences are still unknown. Here we show that in Ramos B-cells, BcR-triggered cell death is associated with an early rise of C16 ceramide that derives from activation of the de novo pathway, as demonstrated using a specific inhibitor of ceramide synthase, fumonisin B1 (FB1), and using pulse labeling with the metabolic sphingolipid precursor, palmitate. There was no evidence for activation of sphingomyelinases or hydrolysis of sphingomyelin. Importantly, FB1 inhibited several specific apoptotic hallmarks such as poly(A)DP-ribose polymerase cleavage and DNA fragmentation. Electron microscopy revealed morphological evidence of mitochondrial damage, suggesting the involvement of mitochondria in BcR-triggered apoptosis, and this was inhibited by FB1. Moreover, a loss of mitochondrial membrane potential was observed in Ramos cells after BcR cross-linking, which was inhibited by the addition of FB1. Interestingly, benzyloxycarbonyl-Val-Ala-dl-Asp, a broad spectrum caspase inhibitor did not inhibit BcR-induced mitochondrial membrane permeability transition but did block DNA fragmentation. These results suggest a crucial role for de novo generated C16 ceramide in the execution of AICD, and they further suggest an ordered and more specific sequence of biochemical events in which de novo generated C16 ceramide is involved in mitochondrial damage resulting in a downstream activation of caspases and apoptosis.

1-phenyl-2-decanoylamino-3-morpholino-1-propanol chemosensitizes neuroblastoma cells for taxol and vincristine.

In this study, we show that an inhibitor of glycosphin-golipid biosynthesis, D,L-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), increases the chemosensitivity of neuroblastoma tumor cells for Taxol and vincristine. At noneffective low doses of Taxol or vincristine, the addition of a noneffective dose of PDMP resulted in 70% cytotoxicity, indicating synergy. Such an effect was not observed for etoposide (VP16). PDMP caused an early (6 h) increase in ceramide (Cer) levels, but the excess Cer was metabolically removed in the long-term (96 h). However, upon incubation with PDMP in combination with Taxol, but not with etoposide, Cer levels remained elevated at 96 h. These results suggest that neuroblastoma cells are normally able to metabolically remove excess Cer, but lose this capacity upon exposure to microtubule modulating anticancer agents (Taxol or vincristine). In addition, PDMP treatment resulted in a decreased efflux of [14C]Taxol and [3H]vincristine from neuroblastoma cells, similar to treatment with PSC833 or MK571, suggesting an effect of PDMP on the transporter proteins P-glycoprotein and/or multidrug resistance protein. PDMP did not further reduce [14C]Taxol or [3H]vincristine efflux in PSC833-treated cells, although it did further diminish cell survival under these conditions. We conclude that a combined administration of nontoxic concentrations of PDMP and either Taxol or vincristine results in highly sensitized neuroblastoma cells. This appears to involve a sustained elevation of Cer levels, possibly in concert with increased drug accumulation.

Inhibition of P-glycoprotein activity and chemosensitization of multidrug-resistant ovarian carcinoma 2780AD cells by hexanoylglucosylceramide.

In the present study we show that neutral hexanoyl-(glyco)sphingolipids inhibit P-glycoprotein (Pgp) activity in human ovarian 2780AD cells. By contrast, hexanoylceramide and the gangliosides GM(3) and GM(2) had no effect on Pgp activity, whereas sphingosine had a stimulating effect. In the case of hexanoylglucosylceramide, inhibition of Pgp activity by was reflected by a regained doxorubicin sensitivity of cells, which were grown in medium supplemented with the lipid. Our results lead to the conclusion that a direct transmodulation of Pgp activity by glycolipids occurs, depending on lipid headgroup structure, which can result in reduced resistance to the chemotherapeutic agent doxorubicin.

Leukemia-induced bone marrow depression: effects of gangliosides on erythroid cell production.

Bone marrow depression is a common feature in hematological malignancies or other bone marrow-involving cancers. The mechanism of this hemopoietic suppression resulting in pancytopenia and especially anemia has not been elucidated. Gangliosides can be shed by cancer cells. Therefore, we investigated the effects of exogenously added gangliosides on erythropoiesis in a human and murine in vitro system. A dose-dependent inhibition of murine colony-forming-unit-erythroid (CFU-E) and burst-forming-unit-erythroid (BFU-E) colony growth was observed. Furthermore the maturation of BFU-Es into CFU-Es was inhibited. The inhibition by gangliosides was not abolished by increasing the dose of erythropoietin (10 U/ml). FACS-analysis studies with human CD34+ cells cultured with gangliosides (GM3), erythropoietin (EPO) and stem cell factor (SCF) demonstrated a strong inhibition on cell growth. This resulted in a significantly higher percentage of immature cells (CD34+/GpA-, 24% vs. 3%), and a lower percentage of mature erythroid cells (CD34-/GpA+, 36% vs. 89%). Under these circumstances the effects on erythroid cell growth were much higher than on other cell lineages. The inhibitory effect of gangliosides isolated from acute lymphoblastic leukemic patients on in vitro erythropoiesis suggests that in vivo hemopoietic suppression might have its origin in the gangliosides present and probably shed by the malignant cells in the microenvironment and plasma. Our results show that gangliosides inhibit erythropoiesis in vitro at several stages of development, by a mechanism involving modulation of the maturation of erythroid cells.

Inhibition of hemopoiesis in vitro by neuroblastoma-derived gangliosides.

Hemopoiesis is disturbed in bone marrow-involving cancers like leukemia and neuroblastoma. Shedding of gangliosides by tumor cells may contribute to this tumor-induced bone marrow suppression. We studied in vitro the inhibitory effects of murine neuroblastoma cells (Neuro-2a and C1300) and their gangliosides on hemopoiesis using normal murine hemopoietic progenitor colony-forming assays. Transwell cultured neuroblastoma cells showed a dose-dependent inhibition on hemopoiesis, indicating that a soluble factor was responsible for this effect. Furthermore, the supernatant of Neuro-2a cultured cells inhibited hemopoietic proliferation and differentiation. To determine whether the inhibitory effect was indeed due to shed gangliosides and not, for instance, caused by cytokines, the effect of DL-threo-1 -phenyl-2-decanoylamino-3-morpholino-1-propanol (DL-PDMP) on Neuro-2a cells was studied. DL-PDMP is a potent inhibitor of glucosylceramide synthase, resulting in inhibition of the synthesis and shedding of gangliosides. The initially observed inhibitory effect of supernatant of Neuro-2a cells was abrogated by culturing these cells for 3 days in the presence of 10 microM DL-PDMP. Moreover, gangliosides isolated from Neuro-2a cell membranes inhibited hemopoietic growth. To determine whether the described phenomena in vitro are a reflection of bone marrow suppression occurring in vivo, gangliosides isolated from plasma of neuroblastoma patients were tested for their effects on human hemopoietic progenitor colony-forming assays. These human neuroblastoma-derived gangliosides inhibited normal erythropoiesis (colony-forming unit-erythroid/burst-forming unit-erythroid) and myelopoiesis (colony-forming unit-granulocyte/macrophage) to a higher extent compared with gangliosides isolated from control plasma. Altogether these results suggest that gangliosides shed by neuroblastoma cells inhibit hemopoiesis and may contribute to the observed bone marrow depression in neuroblastoma patients.